The permafrost regions of the Tibetan Plateau are distributed in the middle latitudes of China,where the gas hydrate is different from the gas hydrate in sea area and polar permafrost area in existing environment and basic characteristics. The lack of effective exploration technology has become the major technical bottleneck that restricts the investigation and evaluation of natural gas hydrate resources in permafrost regions of China. Supported by National 863 Program, Special Research Project of Ministry of Land and Resources and National Special Project of Gas Hydrate, the authors studied the development of key technologies of natural gas hydrate exploration in the permafrost regions of China. The prorate exploration methods for China's terrestrial gas hydrate exploration in permafrost areas have been initially established, which include the high resolution seismic method, the audio magnetotelluric sounding technology, the ultra-deep ground penetrating radar (GPR) technology, geochemical exploration technology and integrated geophysical logging technology. The seismology and electromagnetism identifications of gas hydrate reservoir have been found, the effective index of hydrate geochemical exploration is optimized, and the logging identification technology and reservoir parameter evaluation technology of gas hydrate reservoir are developed. An effective combination method for geophysical and geochemical exploration and a comprehensive geophysical and geochemical exploration model have been tentatively established in permafrost regions. The favorable area of gas hydrate accumulation is predicted, and several gas hydrate reservoirs have been found in the well suggested by the authors, which shows that the validity of methods proposed by the authors are tentatively tested and applied. The research results have great significance for promoting the progress of gas hydrate exploration and for supporting the exploration and development of natural gas hydrate resources in permafrost regions of China.

Gas hydrate in Sanlutian area of Qilian Mountain is different from that of marine gas hydrate, and hence it is very urgent to study geosicience and exploration model. According to the theory of petroleum system and the achievements of geology, geography, geochemistry and drilling, the authors created a four-in-one geological model for gas hydrate deposit. Based on the summary of geophysical and geochemical exploration in recent years, the authors further established a model of gas hydrate exploration in permafrost region. Various geochemical indicators show top anomaly, annular anomaly and negative anomaly. Continuous distribution of high resistance was detected by audio magnetotelluric survey (AMT), the feature of ground penetrating radar is high-speed electromagnetic wave, slow attenuation and obvious reflected signal along bottom boundary of permafrost. The seismic indicators of ore-controlling fault find expression in such phenomena as faulting, termination, distortion, mutation or bifurcation in accordant axis of reflection wave, and the feature of AMT is electrical discontinuity. The reflected signal in gas hydrate bed is low velocity, high frequency and weak amplitude. The logging response is high resistivity, low acoustic time, low natural gamma and low density.

In order to cooperate with the exploration of natural gas hydrate resources,the authors conducted the validity of the experimental study of high precision seismic reflection method in Muli area for the detection of natural gas hydrate in 2010.192 receive channels,2 m group spacing,8 m shot spacing,and 24 folds were used in reflection seismic survey.60 Hz geophone string (with 6 geophones) each received channel received seismic wave in the form of point array.Excitation sources were explosives, shot depth was 3~4 m,and shot charge was 1.2~2.4 kg.The seismic sections obtained by the method possessed high S/N ratio and resolution as well as obvious structural form characteristics.On the basis of a comprehensive analysis and interpretation of reflection seismic,well logging and geological data,the enrichment zones of natural gas hydrate were explained and validated borehole location was determined. The detection results were verified by DK9 borehole drilling in 2013.The gas hydrate reservoirs with large thickness were found 450m southwest of DK3 borehole.Experimental results show that it is difficult to detect natural gas hydrate directly by seismic method in the lower gas hydrate content's area,but through a comprehensive analysis of geophysical,geochemical and geological data in combination with drilling data,the enrichment zones of natural gas hydrate can be detected by seismic exploration.

The presence of gas hydrate and the lithology of reservoirs in Muli area are significantly different from those in the sea area or polar permafrost zone.For analyzing the logging response characteristics of the hydrate reservoir in the study area,the hydrate storage layer identification method is of great significance for hydrate exploration in this area.In this paper,the characteristics of well logging and the causes of hydrate logging were discussed by using conventional logging and ultrasonic imaging logging data, and the characteristics of reservoir logging parameters were calculated.The hydrate recognition was carried out in the study area.Some conclusions have been reached:(1) The apparent resistivity and natural gamma ray are sensitive to the pore hydrate, and the apparent resistivity and the longitudinal wave velocity are sensitive to the fissure hydrate.(2) The thick hydrate of the geological catalog could be identified,with the identification of some suspected hydrate reservoirs,but a small amount of drilling thin layer of hydrate failed to identify the analysis of the reasons for the low hydrate content,resulting in non-obvious logging response characteristics;(3)10 wells in the study identified hydrate and suspected hydrate anomalies,in which the cumulative thickness of the pore-type reservoir was 89m,and the cumulative thickness of the fractured reservoir was 151.6m.Therefore, the fractured hydrate reservoir is the main research area.In summary,the geophysical logging method has a good response to the hydrate reservoir,and can be effectively applied to the reservoir identification of all the boreholes in the study area.

Seismic exploration of natural gas hydrate in permafrost regions in land field is a new field of energy exploration by seismic methods.In order to study the seismic attributes of natural gas hydrate in land field,the authors used the methods of seismic reflection and synthetic seismogram analysis with high precision,and then analyzed the seismic recordings for seismic attributes.The results of experimental research show that the seismic reflection wave group formed by formation of natural gas hydrate has the characteristics of higher frequency and weaker amplitude,and the synthetic seismogram and measured seismic profile are consistent well with each other.In the fluid model,the slope of the curve is positive with the amplitude of reflected waves from the top interface of the reservoir varying with the incident angle,and the slope of the curve is negative in the bottom interface.In contrast,the curve characteristics of the consolidation skeleton model are just the opposite.On the sections of instantaneous amplitude and instantaneous frequency,the amplitudes of frequency properties of reflected waves from the hydrate formation are consistent with those of seismic profiles.On the instantaneous phase section,the phase of reflected waves in hydrate formation has no obvious change,which indicates that the instantaneous phase properties do not reflect hydrate.The corresponding frequency of reflected waves in the high value regions of the energy half-time section is relatively high,which indicates that the energy attenuation is relatively strong when the reflected waves pass through the hydrate formation with a high degree of fragmentation.In conclusion,the results of this study will provide technical support for the seismic exploration of natural gas hydrate in China's permafrost area.

As an important part of the earth’s terrestrial carbon cycle, the Tibetan Plateau has become a hot place of warmhouse gas emission. The effect of gas hydrate exploration on ecological environment deserves much attention. In this paper, the authors studied the flux and isotope of subsurface methane in gas hydrate area of the Qilian Mountain. Some conclusions have been reached: 1. Methane emission from alpine steppe and alpine meadow shows seasonal features. The maximum emission value is 19.2 mg/m²·h and the maximum absorption value is -108 mg/m²·h, demonstrating the role of carbon sink. 2. Methane isotope data show that there exist a large number of microorganisms in the active layer of permafrost region. The methane in 10~30cm is the cause of microorganism, which is relatively active in summer and inactive in winner. The metabolic process of microorganism changes the oxidation-reduction of methane, and bacteria addicted to methane leads to the emission of methane. 3. As for the occurrence state of gas hydrate and the way of exploration, the phenomenon of blast increase of methane in near-surface atmosphere does not appear. 4. The emission of methane is influenced by many factors, and hence the study of temperature, moisture and PH value of soil needs further research.

Next generation sequencing was used in the study of near-surface microbial communities located in gas hydrate well DK1, DK9 and dry well DK6, Muli permafrost zone, to discover the influence of light hydrocarbon microseepage on near-surface microbial community. Compared with dry well DK6, microbial communities in gas hydrate wells changed, especially the hydrocarbon oxidizing microorganisms showed more abundance in gas hydrate well. Furthermore, the predominant hydrocarbon oxidizing microorganisms in gas hydrate well DK1 were alkane oxidizing, and those in well DK9 were methane oxidizing, in accordance with the different gas components in the gas hydrate wells. On the other hand, there were no predominant oxidizing microorganisms in dry well DK6. Besides, selective culture using butanol as the only carbon source also got a number of popular hydrocarbon oxidizing microorganisms, which could be used as the indicators of light hydrocarbon microseepage related to gas hydrate. Above all, the microbial communities, especially both uncultured and cultured hydrocarbon oxidizing microorganisms, showed detectable response to the gas hydrate underground, confirming the feasibility of microseepage based microbial exploration technology used in Muli permafrost gas hydrate survey.

Wetland is an important part of global carbon cycle. As the largest wetland in China, the Tibetan Plateau is a hotspot in the research on wetland carbon cycle. In this paper, the authors evaluated the effect of gas hydrate exploration on wetland carbon cycle in Muli Basin, Qilian Mountain, the Tibetan Plateau. By using geochemical methods such as top gas and acidolysis hydrocarbon, the authors measured the values of the acidolysis hydrocarbon in 2009, 2011, 2013, which were 15.55 μL/kg, 13.73 μL/kg, and 12.82 μL/kg respectively, with no significant increase. The top gas in 2009, 2011, 2013 were 4.49 μL/L, 6.16 μL/L, and 19.22 μL/L respectively, showing increasing trend. The distribution characteristics analysis shows that top gas has short time efficiency, and can not be used to indicate the increase of hydrocarbon gas. The results show that the effect of gas hydrate wells has the features of wide range and low strength on regional carbon cycle system. The environment impact is less and in the controllable range.

The elastic properties of gas hydrate are obviously different from those of the surrounding rocks.The traditional acoustic imaging method using acoustic approximation cannot accurately describe the elastic properties.The imaging results of elastic wave migration method have low resolution,weak energy in deep regions,imbalance amplitude, low frequency noise and acquisition footprint.For this reason,the authors have developed an elastic least-squares inverse time migration method.In this method,an accurate migration operator and demigration operator in the framework of the elastic least squares theory are derived.To improve the computation efficiency and save memory,the authors introduced the multisource elastic least-squares migration method based on encoding technique,in which multiple data are added into a supershot and the encoding technique is used to suppress the crosstalk.The effectiveness and superiority of the proposed method for gas hydrate reservoir imaging are verified by the simulation results of two gas hydrate models.

As the low frequency ground penetrating radar acquisition time window is large, the influence of signal attenuation and dispersion on deep strata is very obvious, the real reflection signals are often accompanied by a lot of noise, and hence the low frequency GPR signal can be regarded as the typical non-stationary signal. The authors used the Hilbert-Huang transform method, which is very effective in non-steady signal processing, to study the weak signal processing method. This means can effectively filter the abrupt change part and noise in the signal, so as to detect and extract the deep weak signal. Using this method to explore the gas hydrate in permafrost zone, the authors successfully extracted the effective reflection signal of low frequency ground penetrating radar signals at the bottom of permafrost and its natural gas hydrate reservoir, thus obtaining good results.

The permafrost area of the Tibetan Plateau has good gas hydrate ore-forming condition and prospecting vista; nevertheless, the lack of effective exploration techniques has become an serious bottleneck affecting the investigation and evaluation of land gas hydrate resources. In this paper, experimental study of the effectiveness of detection of gas hydrate by the audio magnetotelluric sounding method (AMT) was carried out, and the results show that the audio magnetotelluric sounding (AMT) method is an effective method for detecting the permafrost-associated gas hydrate. Permafrost is well developed in the study area, but the thickness varies greatly, which has a certain control over gas hydrate formation. Gas hydrate orebodies show three characteristics in electrical property. These characteristics can be used as an identification markers for gas hydrate formation. The hydrate accumulation is controlled by reverse thrust fractures (F₁ and F₂) on the southern margin of the depression. The fault zone is not only a gas migration channel but also a gas hydrate accumulation space. The research results have positive significance for promoting land gas hydrate exploration technology and resources investigation and evaluation in China.

The field measuring test of free-gas methane was carried out in Muli known hydrate region of Qinghai through west portable soil flux measuring instrument and G2132-iAnalyzer Methane-carbon isotope analyzer. The spatial distribution of free-gas methane characteristic values and geochemical anomaly reveals that there exist good free-gas methane geochemical anomalies above the hydrate deposit, while the spatial distribution of the anomalies is controlled by ore-controlling fractures and underground mineral resources, showing corresponding relation. The research on organic geochemistry and carbon isotope indicates that the gas hydrates in Qilian permafrost region is of thermal origin. The free-gas methane field analysis shows that the free-gas methane geochemical anomaly is mainly of thermal and microorganism origin, and the free-gas methane experiment in winter can reduce the interference of the surface microbial action. The study results indicates that the free-gas methane field measuring technique is applicable to the natural gas hydrate exploration, which can not only indicate the hydrate deposits but also serve as the mutual supplement to other hydrates geochemical exploration techniques, with the best sampling time being winter.

The Qilian Mountain is the first area of finding natural gas hydrate in the middle latitudes of the earth. The logging curve stratification is the first important work of logging interpretation, which is of great significance for the evaluation of gas hydrate logging. So far, natural gas hydrate has been found in many wells of scientific experimental drilling in the Muli area. Based on the logging data of the hydrate drilling wells in the Muli area, the authors used the activity method, the extreme variance method and the Walsh transformation method for automatic stratification. According to the basic principle of automatic stratification, different parameters were used for automatic stratification. A comparative study shows that the stratification effect of the activity method is the best, the stratified effect of the extreme variance method is poor, and the Walsh transform method has the worst stratification effect. The stratification results of the activity method are in good agreement with the lithologic data, thus laying the foundation for the lithologic identification and logging evaluation.

The geological background in Muli area is very complex due to the well-developed faults,fractures and lithologic difference of gas hydrate-bearing formations,which makes it more difficult to identify the lithology with well logging data.Through the analysis of logging response characteristics,the differences between lithologic characters can be found so as to recognize the lithology that contains gas hydrate by utilizing histogram and cross-plot methods.According to natural gamma ray (GR),resistivity (RT),acoustic travel time (AC),and compensated density (DEN),which are sensitive to gas hydrate,the gas hydrate-bearing formation and lithology can be classified by using Bayesian discriminant and BP neural network.The identification results are consistent with the core data,the gas hydrate-bearing lithology is recognized accurately,which can provide some reference for the exploration of gas hydrate.

As an important geophysical exploration means, the well-logging plays an important role in quantitative evaluation/assessment of natural hydrate. With more and more natural gas hydrate being explored in Muli area, a calculation fit for the local shale content, water saturation, porosity and other reservoir parameters is necessary, which is significant for the reservoir optimization and source assessment. Based on the evaluation model of gas hydrate frequently used in the literatures both in China and abroad, the authors consulted the well-logging data of Muli area processed through environment adjustment and standardization, and calculated quantitatively the porosity, saturation and other parameters. With core testing and core electricity data, the authors also amended and improved the parameters calculation models, and analyzed the advantages and disadvantages of different well-logging interpretations applied in gas hydrate reservoir calculation. The results show that the shale content was obtained through natural gamma, the porosity was obtained through gas density resolved by hydrate and had less impact, and saturation was obtained by Indonesia's formula, amended equation and Archie's formulas according to different hydrate values.

Natural gas hydrates are mainly distributed below the seabed and in the permafrost zones.There are obvious characteristics for the impedance of the sea gas hydrate.Nevertheless,very insufficient research has been focused on the impedance characteristics of the land gas hydrate.In this paper,the research aimed at the land gas hydrate in Muli area of the Tibetan Plateau.The authors used the constrained sparse-spike method to invert the impedance with 2D reflected seismic profile which crossed the DK-9 well.The authors analyzed the features of the impedance profile of the four layers that contain gas hydrate.The results show that the impedance of land gas hydrate reservoir is on the whole the same as things of the surroundings.There are a lot of difficulties in detecting the land gas hydrate with wave impedance individually,and hence the reservoir should be detected in combination with other geological and geophysical data.

Obtaining high-quality original data is a key step. Especially in the "dead band" (500 ~ 5000 Hz), the signal of the AMT field source which is the natural electromagnetic field is weak and the apparent resistivity and phase curve are distorted in the dead band. To obtain high-quality data in the permafrost region of the Tibetan Plateau, the authors carried out AMT acquisition experiments of the natural gas hydrate exploration. These experiments included five acquisition parameters, i.e., the acquisition range of time, the acquisition time length, the weather, the electrode length and the density of measuring points. The experiments show that, for the purpose of obtaining high-quality AMT data, the acquisition time range should be selected from the afternoon to the evening, the acquisition time length should be more than 40 minutes, a smaller electrode length (eg. 50~100 m) should be chosen, and the cloudy days are favorable. In addition, the use of high density of measuring points is helpful to building an accurate electrical model.

As the cover of natural gas hydrate deposits in terrestrial permafrost region,the distribution of natural gas hydrate is closely related to the permafrost.It is a new attempt to find the natural gas hydrate by means of seismic method.In order to detect frozen soil effectively,the authors used the method of seismic reflection with small group distance,small shot interval,small offset and high coverage because the permafrost thickness is relatively thin in permafrost region of the Tibetan Plateau.Due to the large difference between the velocities of unconsolidated sedimentary strata with and without freezing,the thickness of frozen soil layer can be reliably detected by analyzing seismic velocity spectrum and reflections reformed by frozen bottom interface.Although the velocity difference of the strata with and without freezing is relatively small for low porosity hard rock sedimentary strata,the impedance difference between the upper and lower strata of the frozen bottom interface will cause the normal reflections to be disturbed and discontinuous.The thickness of the hard rock sedimentary frozen strata can be explained according to the seismic velocity spectrum and the discontinuous disturbance of the tilted reflections.

It is an attempt to detect natural gas hydrate by using shallow seismic technology in permafrost regions of the Tibetan plateau.When the vibrator encounters the locally undulating surface,the ground surface cannot be effectively coupled with the vibrator's excitation plate,and the excitation effect becomes worse.Conducting an empty vibration before the data acquisition of each physical point is helpful to improving the coupling degree between the surface and the seismic vibrator.On the other hand,the interface between the shallow surface low velocity melting layer in summer and the lower high velocity layer is a strong reflection interface.The reflective interface produces so-called energy shielding problem.In winter,the problem of energy shielding can be avoided,but the shallow surface high speed permafrost often causes the vibrator excitation resonance.The harmonic interference in seismic records seriously reduces the quality of seismic acquisition.Under the condition that the surface is close to the "half rigidity",the source excitation not only avoids the energy shield formed by the melting of the low velocity layer on the surface but also avoids the harmonic interference caused by the high velocity layer.The corresponding denoising technique can effectively suppress the various kinds of interference on seismic record and increase signal-to-noise ratio of seismic record.

Previous studies show that frozen soil and fault are important reservoir-forming factors of natural gas hydrate in land-based frozen area. In order to study the fault and frozen soil distribution situations of natural gas hydrate in Muli area, the authors investigated distribution characteristics of frozen soil layer and fault structures of the study area through electromagnetic test. Based on electrical characteristics of resistivity profile of the test area in combination with geological setting, the authors recognized five groups of fault layers in the study area, with four of them possessing the biggest development scale. Frozen soil layer development area within the study area can be divided into 3 sub-areas, and fault layer structure development area can be divided into 4 sub-areas. The authors also analyzed the relationship between natural gas hydrate reservoir-forming condition, frozen soil layer and fault layer structures. By combination of frozen soil layer with area of developed fault layer structure, the authors infer that the east part from F_3-3 to F_3-5 and the place near F₄ fault layer are natural gas hydrate reservoir-forming favorable areas.

Halahu depression in the south of Qilian Mountain is a key region in gas hydrate exploration.The results of seismic exploration show that the shallow lateral velocity reversion discloses that the frozen zone is well developed,and seismic reflection section and three-instantaneous information reveal that Halahu Depression is a composite basin:After the Indosinian orogenic eruption and the Himalayan movement,the Late Paleozoic-Mesozoic residual basin which was developed on the crystalline base of the Caledonian phase further formed as a foreland basin deep in the south and shallow in the north,with the deposition of Cenozoic loose sediments about 300~1200 m in thickness.Under the influence of complex tectonic movement,the strata were severely damaged;in addition,the quality of hydrocarbons source rock was also poor,the formation time of the permafrost was too late to preserve natural gas because the early faults could lead to gas escaping and regional cap layer was nonexistent.In summary,applying hydrate mineralization theory,the authors consider that the metallogenic potential is poor.

South Qilian basin has become the key area of natural gas hydrate survey in the mid-latitude permafrost since natural gas hydrate was discovered in Halahu Depression. The 1:100 000 geochemical survey was carried out to provide evidence for prospective prognosis of natural gas hydrate in Halahu Depression in 2014. Natural gas hydrate and oil-gas geochemical indexes were analyzed in 3000 km² survey area, with the sampling density being 1 sampling site per 2 km². Geochemical anomalies (>30km²) similar to Sanlutian hydrate despoit were delineated, where the abnormal combination of acidolysis methane, acidolysis aridity coefficient, butane isomerization rate, headspace methane and fluorescence 320 exhibited compositional zoning. Further studies show that condensate and coal-type gas were the main source of hydrocarbon gas, and the development of abnormal area was beneficial to the formation of the depth of natural gas hydrate. The results of the survey show that the geochemical technique is suitable for the exploration of natural gas hydrate in the mid-latitude permafrost, and could delineate the prospective area of natural gas hydrate.

On the basis of researches on the hydrate accumulation conditions of permafrost, source rock and temperature-pressure of the hydrate-discovered Muli Sag, geological analogue methods were applied to analyzing the development and occurrence potential of natural gas hydrate in the Hala Lake Sag. The results show that the Hala Lake Sag and Muli Sag have the same permafrost thickness and consistent temperature-pressure conditions, which indicates that the Hala Lake Sag basically possesses the environmental conditions for the gas hydrate formation. With regard to sedimentary evolution, material composition of the permafrost and hydrocarbon gas preservation, they display notable differences. Formations in the Muli Sag are well developed, the permafrost layer is primarily sedimentary rock, and distinct geochemical anomalies were detected. However, a depositional hiatus occurred between the Carboniferous and Jurassic in the Hala Lake Sag, the Triassic petroleum system was destroyed, and the Quaternary sediments are the main materials of permafrost, with poor capping capacity. Deeply buried Permian source rocks could provide hydrocarbon gases for gas hydrate formation. It is inferred that the local uplift of the Hala Lake Sag is a favorable region for the formation of gas hydrate.

The Hala Lake depression in Qilian Mountain has similar conditions for the discovery of the gas hydrate in the Qilian area, which is the key area of the hydrate exploration in the permafrost region of the Tibetan Plateau. By analyzing 45 sections in Hala Lake and conducting audio frequency magnetotelluric survey with total length of 606.4 km, the depression structure and distribution of frozen soil were studied for their electrical characteristics. According to the east and west distribution electrical characteristics of Hala Lake area, the authors mainly analyzed structural features of the east part and the west part of Hala Lake. It is shown that the inner depression testing area of Hala Lake can be divided into 3 depression areas and 2 upwelling areas. The authors also analyzed the distribution and scales of depression areas and 2 upwelling areas. 26 fractures were recognized within the depression testing zone of Hala Lake. These fractures mainly strike in NW—SE direction. The main controlling fractures in the testing area were also discussed. The authors summarized the development characteristics of frozen soil in Hala Lake area and analyzed the main factors influencing the development of the frozen soil. According to the abnormal structure, frozen soil and geochemistry of hydrocarbons, the authors indicate the favorable areas for natural gas hydrate exploration.

Permafrost regions are widely distributed in Hala Lake Basin in the Southern Qilian Mountain,and the basin is considered to be a favorable area for the accumulation of the natural gas hydrates with a great potential.The distribution of permafrost plays an important role in gas hydrate accumulation. The bottom of permafrost on the southeastern margin of Hala Lake was detected by AMT, and the factors affecting the permafrost thickness were analyzed. The detection results are as follows: It is found that using AMT to obtain the permafrost thickness is effective in the study area.Most of the permafrost thickness is between 30 meters and 130 meters, and the distribution of permafrost has the phenomenon that the central and the northwestern parts are thicker whereas other parts are thinner. In particular, the permafrost thickness of the areas including south-central area, central area and northwestern area is more than 80 meters, thus providing good conditions for the cap of the gas hydrate concentration. The main factors for the thickness of permafrost are terrain, slope and surface runoff. In addition, faults and underground water content are also factors influencing the thickness of permafrost .The conditions such as the highlands, the shady side, the non-runoff segment or the non-tectonic fault zone can facilitate greater thickness of permafrost.

The natural gas hydrate in land permafrost of Halahu area in Qinghai is shallowly buried,and seismic effective signal is often submerged by the noise.Reflection signal related to the formation and hydrate can be effectively separated from the complex seismic wave,which is the foundation of seismic detailed interpretation and gas hydrate prediction.In this paper,the field data in Hahahu area are firstly analyzed,and the type and characteristics of the noise are summarized.Then,signal extraction flow is made.Some targeted technologies,such as noise attenuation,amplitude compensation,deconvolution and stack,are used for the pre-stack gathers so as to effectively suppress all kinds of noise and keep the reflected signal to the greatest extent.Seismic resolution,S/N ratio and fidelity of the results are enhanced,which help a lot for the seismic interpretation and gas hydrate prediction.

Mohe basin is the main permafrost distribution area in China. The performance of experimental geochemical prospecting methods in the northwestern permafrost area of Mohe basin detected acidolysis hydrocarbon and adsorbed hydrocarbon in shallow soil, and the adsorbed hydrocarbon has relatively high value, with the average methane value being 12.48μL/L and the maximum value being 49.93μL/L,whereas acidolysis hydrocarbon has relatively low content. The results of three-dimensional fluorescence spectra show that hydrocarbon gas exhibits characteristics of condensate oil, which is pyrolysis gas, indicating that the deep source gas migrates to the surface and manifests microscopic geochemical information. Combined with the permafrost, temperature and pressure conditions, it is shown that the deep permafrost area probably has oil gas and natural gas hydrate.

For the purpose of supporting gas hydrate resources exploration, audio frequency magnetotelluric sounding was carried out in the northern part of Mohe basin in Northeast China. Through a comparative analysis of two-dimensional geoelectrical section interpretation result of audio frequency magnetotelluric sounding and planar resistivity characteristics, the authors conducted division of electrical property units and faults, and perfected "one uplift and one depression" structural framework in northwestern littoral uplift area of Mohe basis, thus forming geological-structural framework of "two uplifts and two depressions, alternate uplift and depression". Spatial distribution characteristics of regional faulted structure were preliminarily found out; F₂, F₄ and F₈ faults somewhat control the secondary structural framework. In combination with physical character of strata and thickness of permafrost distribution, the authors made an analysis of the favorable conditions for the formation of gas hydrate reservoirs and pointed out that the area to the south of Beihongcun, i.e., electrical property unit ③ is favorable for the formation of gas hydrate reservoirs.

Mohe basin has favorable reservoir-forming prospect for gas hydrate, and the understanding of the permafrost development situation in the basin is very important for evaluation of gas hydrate resources potential and optimization of favorable areas. One of the purposes of carrying out audio frequency magnetotelluric sounding profile measurement in Mohe area of Northeast China is to find out the buried depth and spatial distribution of permafrost in the working area. Based on an analysis of the electrical property characteristics and high resistivity features of the permafrost, the authors made thickness recognition and division. The exploration results show that the permafrost in northern Mohe basin exhibits island-shaped distribution, with characteristics of thick in the north and thin in the south, thin in the west and thick in central and east parts, with the maximum thickness being 120m and the average thickness being 40~80, which demonstrates that the study area has favorable permafrost conditions for the formation of gas hydrate resources.

The thickness of permafrost is an important factor for the formation and distribution of gas hydrate in permafrost areas.Recognizing permafrost thickness accurately is significant for gas hydrate resource exploration in permafrost areas of Northeast China.Based on the characteristics of permafrost from temperature logging,the authors established the geothermal temperature model and carried out research on temperature logging data from the nine gas hydrate exploration holes.Some conclusions have been reached:(1) The temperature logging curve has an obvious inflection point at the bottom floor.The slope of the temperature logging curve above the inflection point is less than that under the inflection point.Using the temperature logging data can accurately recognize the permafrost bottom floor.(2) Too short hole over-flow water and drilling fluid equilibrium time can influence the accuracy of temperature logging data.The length of sunshine duration influences the development depth of permafrost.

Gemucuo surveying area is on the northern margin of central uplift belt in Qiangtang basin.The results of regional geological survey of natural gas hydrate indicate that the area is provided with relatively good external gas source condition,transport tectonic condition,permafrost condition,reservoir and storage condition for generating gas hydrate and the delineation of the gas hydrate favorable block.In order to study the seismic method suitable for detecting gas hydrate and to better search for the distribution area of gas hydrate in the permafrost area,the authors carried out exploration work and contrast experiment of shallow and mid deep seismic reflection.The test results show that the characteristics of geological structure and stratigraphic distribution reflected in the shallow and mid deep seismic reflection profile are similar,except for that fact that the resolution is higher and the detection depth is shallower on the shallow seismic section.According to the results of the seismic exploration,the geological structure map of the survey area is made.On the basis of the geological data and other geophysical and geochemical data in the surveying area in combination with the attributes of reflection from known natural gas hydrate reservoirs in Muli surveying area,favorable distribution areas of natural gas hydrate are predicted,and the proposal of verification hole position is put forward.

Gemucuo area is located on the northern margin of the central uplift belt in Qiangtang basin.The results of regional geological survey of the area show that the area is a favorable area of gas hydrate formation.In order to detect the gas hydrate,the authors carried out high resolution seismic reflection detection test in the area.High quality original seismic data were obtained in the process of seismic data acquisition by using such means as the observation system of Intermediate excitation and bilateral symmetry and the receive mode of long spread and shot trace intervals as well as the excitation mode of large tonnage vibrator.In seismic data processing,according to the characteristics of permafrost area in the plateau,the authors conducted a large number of tests for parameters,and used a combination of various means of treatment and the reflection seismic time profiles,thus obtaining high signal-to-noise ratio.The seismic reflection wave group in the seismic time profiles clearly reflects the features of subsurface geological structure.The authors investigated and delineated the favorable target area for gas hydrate accumulation in Gemucuo.The test results show that the high resolution seismic reflection detection technology is a very effective means for detection of natural gas hydrate,and hence it can serve as a reference technology for the seismic exploration of natural gas hydrates in permafrost regions in plateau in future.

Since the discovery of natural gas hydrate in Muli area of Qinghai Province for the first time in 2008, a series of geophysical and geochemical exploration work has been done. However, the fact that the gas hydrate reservoir in this area has features of low porosity and low permeability has brought many challenges to exploration work. To obtain the electrical response characteristics of the natural gas hydrate models in the frozen soil area, the authors conducted the AMT forward modeling of the gas hydrate in this paper. Based on the thin layer distribution characteristics of gas hydrate and the distribution range of the logging resistivity values of gas hydrate, the authors constructed two types of models, i.e., sandstone and mudstone models. Through the AMT forward modeling and inversion studies of the two models, the electrical response characteristics of the two models were obtained. The results are as follows: (1) When the ratio of the thickness of the hydrate layer to the depth of the hydrate layer and the ratio of the resistivity of the hydrate layer to the resistivity of surrounding rock meet certain conditions, both the sandstone models and the mudstone models can be used to distinguish the hydrate layer by 1D inversion curve. (2) In the hydrate stability zone of Muli area, the resistivity value of the hydrate layer is equal to 400 Ω·m or 200 Ω·m; when the ratio of the thickness of the hydrate layer to the depth of the thin hydrate layer is equal to 10%, the one-dimensional inversion can be used to distinguish the hydrate layer, and when the ratio of the thickness of the hydrate layer to the depth of the thin hydrate layer ≤5%, the one-dimensional inversion can not distinguish the hydrate layer.

In order to study the feasibility and key parameters of applying low frequency ground penetrating radar (GPR) to detecting gas hydrate in permafrost area,the authors used forward modeling of detection depth,resolution and reflection feature analysis of the low frequency ground penetrating radar in Tibetan Plateau permafrost environment modeling.Firstly,the relationship between the maximum detection depth of theoretical ground penetrating radar and the transmitting frequency,the resistivity of the underground medium and the dielectric constant of the GPR is determined by the radar range equation and,according to the theory of electromagnetic wave reflection,the system gain required for detecting the depth of gas hydrate in permafrost zone was calculated;Secondly,the resolution of low frequency GPR at large scale (200 meters) is determined by resolution calculation and simulation;Finally,two dimensional forward modeling experiments are carried out by time domain finite difference (FDTD) method,and the reflection characteristics of GPR signals at the bottom of the permafrost and the top and bottom boundaries of gas hydrate are obtained,which provide useful information for the processing and interpretation of field data.The results show that the low frequency GPR with a center frequency of less than or equal to 15 MHz and a system gain of more than 165 dB can meet the requirements of the detection of gas hydrate reservoirs in the permafrost areas with high surface resistivity.The resolution calculation and simulation results show that the low frequency ground penetrating radar can reach the generalized resolution of 1% at the depth of 200 meters under certain conditions,and that the GPR signals have obvious strong amplitude and frequency mutation characteristics on the bottom of permafrost and the top and bottom of gas hydrate.The theoretical calculation results show that low frequency ground penetrating radar is suitable for detecting natural gas hydrate in permafrost zone.

The low frequency ground penetrating radar system is equipped with unshielded antenna and poor anti-jamming capability. For the low frequency ground penetrating radar with a center frequency of 15MHz, the authors designed and manufactured three different sizes of shielding devices (length×width×height: 12 m×12 m×5 m, 11 m×11 m×5 m, 9.6 m×9.6 m×5 m) with the iron wire mesh as the shielding material. The experimental research on shielding interference of the receiving antenna and the transmitting antenna of low-frequency ground penetrating radar was carried out respectively. When electromagnetic shielding experiment was carried out separately for the receiving antenna, the shielding devices of the three sizes showed better shielding effect, and the optimal size was 11 m×11 m×5 m. When the electromagnetic shielding experiment was carried out separately for the transmitting antenna, only the shielding device of 11m×11m×5m was the most effective, and the shielding effect of the other two devices was relatively poor. In contrast, it was better to add a separate shielding device to the receiving antenna than to add a separate shielding device to the transmitting antenna. The experimental results have laid a solid foundation for future research and development of the practical shielding device of low frequency ground penetrating radar.

The concentrations of ions will have a certain effect on the change of unfrozen water content of permafrost in the frozen-thaw process and therefore on the nuclear magnetic resonance response signal of frozen soil. In order to study the influence of the ion concentration on NMR signal, the authors made the samples with the mixture of NaCl solution and Malan loess in which NaCl values are different and initial water values are the same. By analyzing the NMR response characteristics of the samples at different temperatures, the authors summarized the influence of the ion concentration on the NMR response of permafrost. The results show that increasing the ion concentration will lead to the increase of the amplitude of the NMR signal, but will not affect the transverse relaxation time, and that, during the frozen-thaw process, the fluid will be affected by the temperature and ionic crystals. Samples of unfrozen water content will change in different temperature ranges, but when temperature is higher than 5℃ or lower than -20℃, unfrozen water content will tend to be stable.

Susceptibility of paramagnetic material changes when the temperature changes, and non-uniform magnetic field will have a certain influence on nuclear magnetic resonance (NMR) signal. In the study of physical properties of frozen soil with NMR, the effects caused by paramagnetic material in frozen soil cannot be ignored. In combination with the studies of physical properties test of frozen soil conducted both in China and abroad, the authors mixed Fe₂O₃ with Malan loess and water to make samples in which Fe₂O₃ values were different and initial water values were the same. Then the authors analyzed the relaxation characteristics of the samples. Through the analysis of the NMR signal amplitude and transverse relaxation time of the samples with the change of temperature, the influence of different paramagnetic values on NMR signal of frozen soil was summarized. The results show that the paramagnetic material can reduce NMR signal amplitude and transverse relaxation time of fluid in porous media significantly. The NMR signal and paramagnetic content meets some empirical formula below 0℃, and the paramagnetic material will cause some errors in the unfrozen water content calculation of the samples.

The exploration of natural gas hydrate in the permafrost area is developed quickly.Several wells containing gas hydrate reservoirs have been found in Muli area.Nevertheless,there is no software log evaluation system in China,which affects the exploration progress of gas hydrate in Muli area.Therefore,based on the geolog,engineering and data sets,the authors developed the logging evaluation system of natural gas hydrate in Muli area by using VB language.The main technology and methods of the system include(1) the pretreatment technology of logging data,(2) the analysis of logging response and lithologic characteristics,(3) the hierarchical and lithologic identification technology,and (4) the calculation and evaluation of reservoir parameters.Combined with the technology and methods,the structure and function of the system are designed.The layered structure is selected,and it contains 3 layers,i.e.basic data layer,middle support layer and application layer.The main functions of the system and technology are realized,and a set of chain process modular and technology flow charts is established.The field application results show that the system can satisfy the exploration processing precision and efficiency of hydrate logging interpretation requirements.The structure of the system is stable,and the function is completed.It achieves the target of the efficient processing and interpretation of gas hydrate logs in Muli area and fills the gaps in related fields.The system could support the exploration and development of natural gas hydrate in Muli area.

In this paper, the fuzzy statistic method was used to forecast the distribution of natural gas hydrate resources in Muli area, with the geochemical exploration data obtained from Muli area as the study object. The results show that it is feasible to use this method to predict hydrate. The prediction area is well correlated with the drilling of the known hydrate, and the six hydrate favorable regions were delineated. The result obtained by the authors is of great significance not only for the optimization of material and geochemical exploration methods, the analysis of abnormal characteristics and the establishment of evaluation indexes, but also for the acceleration of the exploration and potential evaluation of natural gas hydrate resources in China's landfill regions.